1. Field of the Invention
The present invention relates to an antenna device and a radio device comprising the same, and more particularly, to an antenna device adapted to use with two frequency bands and a radio device comprising the same.
2. Related Art
FIG. 6 shows an antenna device adapted to use with two frequency bands, which is a prior art of the present invention. In the antenna device 40 shown in FIG. 6, two dipole antennas 41, 42 of which the resonant frequencies are different, are arranged at an interval and connected to one signal supply 43. The antenna device can be so constructed as to be adapted to use with two frequency bands by arranging the two dipole antennas having different resonant frequencies as described above.
Furthermore, another antenna device which is also a prior art of the invention is shown in FIG. 7. Its basic arrangement is disclosed in Japanese Unexamined Patent Publication No. 7-12832. It should be noted that this antenna device was arranged in order to be used with a wider frequency band rather than with two frequency bands.
An antenna device 50 shown in FIG. 7 comprises a ground board 51, and an inverted F-shape antenna 52, and a microstrip antenna 53 arranged on the ground board 51. The inverted F-shape antenna 52 includes a first radiation conductor 52a having a rectangular shape and a length substantially equal to a quarter-wavelength, of which one end is open and the other end is connected to the ground board 51 through a first connecting conductor 52b whereby the other end functions as a ground end, and a feeding conductor 52c provided in the vicinity of the ground end of the first radiation conductor 52a and having one end connected to the first radiation conductor 52a. The microstrip antenna 53 includes a second radiation electrode 53a having a rectangular shape and a length substantially equal to a quarter-wavelength, of which one end is open and the other end is connected to the ground board 51 through a second connecting conductor 53b whereby the other end functions as a ground end. The open end of the second radiation conductor 53a of the microstrip antenna 53 is so arranged that it is positioned near to the open end of the first radiation conductor 52a of the inverted F-shape antenna 52, and the sides of both open ends are in parallel with each other. The resonant frequency of the microstrip antenna 53 is set to be close to that of the inverted F-shape antenna 52. A signal supply 54 is connected to the feeding conductor 52c of the inverted F-shape antenna 52, while the feeding conductor 52c is insulated from the ground board 51.
According to the antenna device 50 configured as described above, a signal, input to the inverted F-shape antenna 52 from the signal supply 54, causes the inverted F-shape antenna 52 to become resonant, and is transmitted to the microstrip antenna 53 through a static capacitance C53 produced between the open end of the first radiation conductor 52a of the inverted F-shape antenna 52 and the open end of the second radiation conductor 53a of the microstrip antenna 53, causing the microstrip antenna 53 to resonate. Thus, the inverted F-shape antenna 52 and the microstrip antenna 53 become double-resonant. That is, the antenna device 50 resonates in a wider frequency band as compared with the inverted F-shape antenna 52 solely. Thus, the antenna device 50 can be operated as an antenna adapted to use with a wider frequency band, as compared with the inverted F-shape antenna 52 solely.
However, according to the antenna device 40 shown in FIG. 6, an unnecessary interference occurs in some cases so that required characteristics can not be obtained, if the interval between the two dipole antennas 41 and 42 is short. In order to reduce the mutual interference between the two dipole antennas to a negligible level, it is required to increase the interval between the two dipole antennas to be at least 0.3 times the wavelength. As a result, this causes a problem that the antenna device as a whole becomes large in size.
Furthermore, according to the antenna device 50 shown in FIG. 7, the frequency band becomes wider to some degree as compared with that of the inverted F-shape antenna solely used, but the antenna device 50 can not be operated as an antenna adapted to use with two frequency bands not overlapped.